1. Field of Invention
The invention relates to a physical quantity estimating apparatus and a tire state determining apparatus. More particularly, the invention relates to a physical quantity estimating apparatus capable of estimating a physical quantity relating to a tire radius, taking into consideration a tire torsional spring and a suspension longitudinal spring of an estimated ideal vehicle model, using a method for estimating a road surface friction coefficient (μ) gradient based on a wheel speed difference between any two wheels, e.g., a rear wheel and a front wheel, and an acceleration and deceleration rate of a vehicle during acceleration and deceleration of the vehicle, and a tire state determining apparatus that determines a tire state such as a tire type using the physical quantity estimated by the physical quantity estimating apparatus, and a determination method of that tire state determining apparatus.
2. Description of Related Art
As related art, a tire identification method for determining the type of tire on a vehicle is known (Japanese Patent Laid-Open Publication No. 9-188114). According to this method, the rotational speeds of four tires fitted on a vehicle are measured, and a ratio (front to rear wheel ratio) of the rotational speed of the front wheels to the rotational speed of the rear wheels is calculated from the measured rotational speeds. The type of tire is then identified by comparing a speed, a front to rear wheel ratio function, which was created based on data from a tire of a known type, to a value of the calculated front to rear wheel ratio.
The tire identification method of the foregoing related art, however, does not take into consideration the difference in the tire radius, and is therefore unable to determine the tire type with high accuracy.